Privilidges of VPS

I have been with Siteground shared plan for a while, and I heard about dream host VPS. I have experience with Amazon EC2. I have a question:

1- How much access do I have to VPS? I know I have root access, but how come I can’t install specific version of php or mysql or any software that I want? In amazon EC2, it’s like the exact local machine, and I had the full access over the system.

2- Does VPS come with pre-bundled with lamp? or any other packages? can I install software updates?

3- I’m seeing many issues with dreamhost servers in this forum, which I hand’t with Siteground? Do VPSs have many service outage, problems? Are they on better servers than shared servers?